Great cormorants (the winter cormorant around Boston) are fatter than gannets at 7.2 lbs (vs. 6.6 for gannets), but have a three-foot wingspan instead of a six-foot one. There's a great cormorant in the center of the photo. The more common double-crested cormorants have a similar wingspan at 33 inches, but weigh only 3.7 pounds.
Great black-backed gulls are pretty big too - 65-inch wingspan and 3.6 pounds. This one's a juvenile.